both is healthy, depending on purpose, quantity, and for what reasons.

for a baby, mother's milk is not just healthy it's essential. For example, a newborn baby within 1-2 months cannot properly ingest food, and will get sick and possibly die if you only give it water and try to feed it.

water is is also essential for most living things, but once again too much later even digested in humans can cause death (it's called Water intoxication)

if you mean for an average adult, then other assumptions need to be made, are they lactose tolerant, are they malnourished in protein, do they have any other diet issues, do they have any auto-immune disease affected by water or milk consumption.

note; there are some cultures who survives on water to supplement the lack of water for all their adult lives.